Abstract: Intracellular mitochondrial enzyme replacement therapy for heart and skeletal myopathy in Barth Syndrome Barth Syndrome is an X-linked disorder resulting from defects in the gene encoding Tafazzin, an acyltransferase that modifies cardiolipin to the tetralinoleoyl form and is essential for mitochondrial respiration. The clinical manifestations of Barth Syndrome include muscular hypotonia, cardiomyopathy and neutropenia. At present, no effective therapy exists for affected individuals. The main issue that this project will address is whether an enzyme replacement strategy in which recombinant tafazzin is linked to a cellular penetrating peptide can promote uptake into tafazzin- deficient cells, restore cardiolipin remodeling, suppress abnormal production of mitochondrial reactive oxygen species (ROS), restore normal mitochondrial respiration, improve heart and skeletal muscle performance and improve neutropenia. This project will utilize a mouse model of Barth Syndrome from Jackson laboratories and initial studies to characterize baseline function in heart and skeletal muscle have been done. Other significant milestones include generation and purification of recombinant tafazzin protein that has been modified to contain a cellular permeability peptide, along with verification of its ability to enter cells and correct both cardiopin remodeling and mitochondrial respiration defects in vitro and in live animals. Other project findings are that tafazzin-deficient myocytes produce ROS at higher levels than control cells, and that exogenous tafazzin protein suppresses this excess ROS generation. In this proposal, elucidation of mechanisms by which recombinant tafazzin engineered for intracellular uptake can enter cells and correct the metabolic defects in tafazzin-deficient cells or animals will facilitate understanding of whether this novel recombinant tafazzin reagent can provide a treatment for cardiomyopathy, skeletal myopathy and neutropenia in Barth Syndrome patients.